1. Field of the Invention
The present invention relates to a particle beam therapy system capable of high precision irradiation for treatment, and more particularly to a particle beam therapy system suitable for using a spot scanning irradiation method.
2. Description of the Related Art
In the recent aging society, a typical one of radiation therapies has attracted attention as one of cancer treatments since the radiation therapy is noninvasive to and has a low impact on human bodies. In addition, after the radiation therapy, the quality of life is highly maintained. Among the radiation therapies, a particle beam therapy system is a promising approach since the system provides an excellent dose concentration for an affected area of a patient. The particle beam therapy system uses a proton or a charged particle beam such as carbon, which is accelerated by an accelerator. The particle beam therapy system includes an accelerator, a beam transport system and an irradiation device. The accelerator such as a synchrotron or cyclotron is adapted to accelerate a beam emitted by an ion source to a level close to the speed of light. The beam transport system is adapted to transport the beam extracted from the accelerator. The irradiation device is adapted to irradiate an affected area of a patient with the beam in accordance with the location and shape of the affected area.
Conventionally, in an irradiation device provided in a particle beam therapy system, a beam is formed by increasing the diameter of the beam by a scatterer and removing an outer periphery of the beam by a collimator in order to irradiate an affected area of a patient with the beam in accordance with the shape of the affected area. In this conventional method, the efficiency of using the beam is low, and unnecessary neutrons tend to be generated. In addition, there is a limitation in matching the shape of the beam with the shape of an affected area of a patient. Recently, there has been an increased need for a scanning irradiation method as a higher precision irradiation method. In the scanning irradiation method, a beam having a small diameter is extracted from an accelerator, and bent by an electromagnet. An affected area of a patient is then scanned by the beam in accordance with the shape of the affected area.
In the scanning irradiation method, a three-dimensional shape of an affected area is divided into a plurality of layers in a depth direction, and each of the layers is two-dimensionally divided into a plurality of portions to set a plurality of irradiation spots. Each of the layers is selectively irradiated with an irradiation beam by adjusting the energy of the irradiation beam in accordance with the depth position of the layer. Each of the layers is two-dimensionally scanned with the irradiation beam by electromagnets. Each irradiation spot is irradiated with the irradiation beam with a predetermined dose. A method for continuously turning on an irradiation beam while the beam spot is moved from an irradiation spot to another irradiation spot is called raster scanning, whereas a method for turning off an irradiation beam while the beam spot is moved from an irradiation spot to another irradiation spot is called spot scanning.
In the conventional spot scanning method, each irradiation spot is irradiated with a beam with a predetermined dose under the condition that beam scanning is stopped, and after the irradiation beam is turned off, the amount of an exciting current flowing in a scanning magnet is adjusted, and then the beam spot is moved to the location of the next irradiation spot. To achieve high precision irradiation for treatment using the spot scanning method, it is necessary to position a spot of an irradiation beam with high accuracy and to turn on and off the irradiation beam at a high speed. Especially, it is necessary to turn off the irradiation beam at a high speed.
To obtain high accuracy of positioning of the irradiation beam spot, a known beam extraction method is used. In the beam extraction method, the size of the circulating beam is increased by a radio-frequency power, and particles having large amplitude and exceeding a stability limit are extracted in order to extract a beam from a synchrotron. In this method, since an operation parameter of an extraction related apparatus for the synchrotron can be set to be constant during the extraction of the particle, orbit stability of the extracted beam is high. Therefore, an irradiation beam can be positioned with high accuracy, which is required for the spot scanning method.
However, it takes a certain time to block the extracted beam after radio-frequency (RF) power for extraction is turned off at the time of termination of irradiation on each spot. Thus, the irradiation during the delay time (delayed irradiation) occurs. It is necessary to reduce the irradiation dose of the delayed extracted beam in the spot scanning method in order to maintain the accuracy of the irradiation dose. Therefore, the beam extracted from the synchrotron is controlled to prevent the beam from reaching an irradiation device by turning on and off a shielding magnet provided in a beam transport system during a movement of the beam spot from an irradiation spot to another irradiation spot. For example, JP-A-2005-332794 discloses that an extracted beam is deflected by a shielding magnet provided in a straight section of a beam transport system and an unnecessary component (that may cause delay irradiation) of the beam is removed by a beam dump provided on the downstream side of the straight section of the beam transport system. FIG. 11 shows the configuration of a conventional particle beam therapy system having a beam interrupting device.
On the other hand, when the cyclotron is used as the accelerator, delayed irradiation may occur. A voltage applied to an ion source is controlled to turn on and off a beam that is to be extracted from the cyclotron. After the application of the voltage to the ion source is stopped upon termination of irradiation on each spot, it takes a certain time to block the beam in order to prevent the beam from being extracted from the cyclotron. To take measures for the above problem, for example, JP-A-2005-332794 discloses a particle beam therapy system (shown in FIG. 11) having a synchrotron, as is the case with a synchrotron used as the accelerator.